This invention relates to a microwave antenna system. More particularly, the invention relates to an antenna system suitable for use in propagation and reception of electromagnetic radiation at microwave frequencies, that is, at frequencies wherein waveguide can be utilized in the transmission or reception of electromagnetic radiation. For the purposes of the present invention, the term "antenna" means a device used with a transmitter for the purpose of radiating into free space (including the earth's atmosphere) periodic electromagnetic radiation or for receiving from free space such electromagnetic radiation. The antenna of the invention is particularly suitable for use in communications satellite applications and also may be used in radar or other applications.
Communications satellites are maintained in a synchronous orbit which is a substantially circular orbit about the surface of the earth and at an altitude and with a velocity parallel with the earth's surface such that the satellite does not substantially change in position relative to a particular point on the surface of the earth. In other words, although the satellite has an orbital velocity, the surface of the earth also is rotating and the angular velocity of the satellite may be made to correspond to that of the earth's surface, thereby, resulting in no relative movement of the satellite with respect to a point on the earth's surface. If this sub-satellite point is located in one of the oceans, such as the Atlantic ocean, antennas mounted on the satellite may be used to receive electromagnetic radiation from, for example, the easterly continent or hemisphere and may be used to retransmit this received radiation, at a different frequency, to receiving stations located in the westerly continent or hemisphere.
The transmission and reception by the satellite is accomplished typically with microwave electromagnetic frequencies modulated as required to convey information between remotely located points on the earth's surface or to transmit to various earth locations information accumulated by equipment aboard the satellite. In the transmission from and reception by the satellite of electromagnetic radiation, a suitable antenna is positioned at or near the focal point of a reflector to obtain an antenna system having high directivity toward, for example, the western hemisphere and a high degree of isolation of this westerly beam with respect to the radiation reflected from the reflector toward the easterly hemisphere. The reflector typically is parabolic in form and may be of the offset parabolic type wherein the antenna system includes a radiation propagating feed source located at the focal point of a usually elliptical portion of the paraboloid surface. The radiation feed arrangement illuminates this elliptical portion of the paraboloid surface, which, in turn, reflects the radiation toward the earth. Preferably, the elliptical portion of the paraboloid surface is offset from the focal point at which the feed device is located so that the feed device does not interfere with the electromagnetic radiation either being received by or transmitted from the reflector portion of the antenna system.
Publications describing satellites, antenna systems using offset and parabolic reflectors, and feed arrangements that may be utilized are available and reference is made to these should further background information be desired.